1. Field of the Invention
The present invention relates to a light emitting device comprising a plurality of pixels each including a light emitting element and a unit for supplying current to the light emitting element.
2. Description of the Related Art
Since a light emitting element emits light by itself, it is highly visible and does not require a back light which is needed in a liquid crystal display device (LCD). Therefore, it is suitably applied to thin devices and not restricted in viewing angles. Because of these advantages, a light emitting device having light emitting elements has recently been drawing attentions as an alternative display device to a CRT and an LCD, and practically applied to an electronic device such as a mobile phone or a digital still camera.
The light emitting device can be classified into a passive matrix type and an active matrix type. The active matrix light emitting device can maintain a current supply to a light emitting element to some extent even after a video signal input, therefore, a larger size and higher definition of a panel can be realized, which is becoming the mainstream in recent years. The specific pixel configurations of an active matrix light emitting device which are proposed are different among manufacturing corporations of light emitting devices, and various technologies with distinctive characteristics have been studied and developed by them. Typically, a pixel includes at least a light emitting element, a transistor for controlling a video signal input to the pixel, and a transistor for supplying current to the light emitting element.
As an obstruction to the practical application of the light emitting device, a luminance decay due to degradation of light emitting materials can be noted. The rate of degradation of the light emitting materials depends on the emission period or the current amount supplied to the light emitting element. Therefore, when a gray scale differs among pixels in each displayed image, the rate of degradation of the light emitting elements vary among pixels, leading to variations in luminance. Patent Document 1 discloses a method for suppressing the luminance decay by maintaining drain current of a transistor for controlling a current amount supplied to a light emitting element constant when it is ON regardless of the degradation of a light emitting layer by operating the transistor in a saturation region.
Patent Document 1
Japanese Patent Laid-Open No. 2002-108285
However, drain current of a transistor operating in a saturation region is largely influenced by small variations in its gate-source voltage (gate voltage) Vgs, which varies luminance of a light emitting element. Therefore, when operating the transistor in a saturation region, its gate voltage Vgs has to be prevented from varying while the light emitting element emits light.
However, when OFF-current of a transistor for controlling a video signal input to a pixel is large, its gate voltage Vgs easily varies with the potential change of a video signal which is input to other pixels. In order to prevent the variations of the gate voltage, it is required that the storage capacity of a capacitor disposed between the gate and source of the transistor is increased or OFF-current of the transistor for controlling a video signal input to a pixel is suppressed low. However, the area occupied by the capacitor is preferably small in order to prevent leak current between electrodes due to the dust or drop in yield. Further, it is quite difficult to optimize the manufacturing process of a transistor so as to suppress OFF-current of a transistor for controlling a video signal input to a pixel low and to increase ON-current of the transistor to charge large capacitance since it requires high cost and time. Further, the gate voltage Vgs of the transistor for controlling a current amount supplied to a light emitting element easily varies with the switching of other transistors or the potential change in a signal line and a scan line due to the parasitic capacitance of the gate of the transistor.